In internal combustion engines, it is essential that the cylinders be completely airtight when the valves are closed to assure efficient fuel consumption and transfer of power. This airtightness is achieved, among other things, by assuring that the valve heads perfectly match the valve seats. Since contact surfaces of the valves and valve seats are subject to wear and other degrading factors that affect the contact surface textures, these surfaces must be modified to re-establish the high quality seal.
It is well known among vehicle mechanics, especially that valve seats can be machined to remove the outer surface of the seat to expose a smooth and uniform contact surface. This machining is accomplished by removing the engine cylinder head and inverting it on a work surface to provide access to the valve seats. A cutting blade is positioned to cut at the desired radius and a pilot which is concentric with the center of rotation of the cutting blade is inserted into the valve guide to center the cutting edge. A spindle attached to a drive motor rotates the blade and pilot to remove the outer layer of the contact surface of the valve seat.
While this procedure would be easy to set up and would provide consistent results if all valve guides were oriented in the same direction, cylinder heads have a wide range of valve guide orientation (angular displacement) within a single head, with two or four valves per cylinder. Thus, it is necessary to provide capability to move both the tool position along the length and width of the cylinder head and the valve guide orientation. In order to achieve this, it is necessary to provide precision movement of a large, heavy piece of equipment.
Probably the best known system providing adjustability of the machining head is that it is manufactured and sold by Serdi, a portion of which is disclosed and claimed in U.S. Pat. No. 4,365,917, issued Dec. 28, 1982, of Pierre Harmand, one of the co-inventors of the present invention. This work-holding device utilizes a machine head with a spherical guide within a spherical guide seat conforming to the sphere. The machine head is mounted on a base and a horizontal plane is provided by moving the system's base along an air cushion. The air cushion provides virtually frictionless motion. Air cushions are also created between the sphere and the sphere seat to allow the machine components (motor, spindle, etc.) to be displaced about the sphere's centroid, changing the angular orientation of the spindle. A closely-tolerenced pilot (within 1/100 mm of the valve guide inner diameter) is introduced into a valve guide and machine head selfcenters with the sphere moving freely within the guide seat. A locking system locks the sphere in place once it is properly aligned. After machining the valve seat, the sphere is unlocked and the pilot is extracted. The spindle is returned to a vertical starting position and the process is repeated for each valve seat of the cylinder head.
A disadvantage of the above system is that the entire machine head is lifted and supported by the air cushion to center the spindle. Since the head moves freely at this point, the significant weight of the head, approximately 440 lbs. (200 kg), develops considerable kinetic energy which must be controlled to insert the pilot into the valve guide, and the pilot must be able to absorb the kinetic energy of the head to hold its movement until all oscillation has ceased and any flexion of the pilot has stopped. In short, the pilot acts as a spring and realigns/centers the machine head until all lateral forces dissipate.
A second prior art embodiment of a valve seat and guide machine provides horizontal travel by way of a movable stage onto which the cylinder head is clamped. The spindle still moves within a spherical housing to provide angular control, however, lateral adjustment is enabled by the stage which is separated by the base of the fixed head system by an air cushion. While this system may have an advantage in that the weight of the machine head does not need to be moved, the combined weight of the moveable stage and the cylinder head is over 330 lbs (150 kg) which means the considerable momentum is generated when the stage is moved. Further, this system has the disadvantage that the head height is fixed relative to the work surface, such that z-axis adjustability can only be attained by modifying the spindle length. Using slides, this can result in frequent spindle modifications and may impact centering and rotational stability due to variable lengths and rigidity of the spindle and the fact that spindles are typically driven from the end farthest from the cutting tip.
Prior art valve seat machining systems have displayed further problems when repeatably cutting valve seats with a three-angle cutter. In this procedure, a large surface is typically being cut, making control especially crucial. Particularly, the machinist must keep the cutter from rubbing the metal once the cut is completed, i.e., when the spindle feed is completed. It is established that the best cut is obtained when the spindle rotates one to three turns without cutting at the end of the cut, in a "polishing" type of process. If the operator fails to bring the spindle back up, since the cutter is still in contact with the valve seat, it will vibrate or chatter, possibly damaging the valve seat and/or the cutting bit. In order to achieve the ideal one to three additional turns to assure that the valve seat is round but still avoid vibration or chatter, the user must be quick at raising the spindle, since at speeds of several hundred revolutions per minute, the desired few additional turns occur within fractions of a second. This makes control and repeatability a significant problem.
In view of the aforementioned inadequacies of the prior art, the need exists for a valve seat and guide machine that can readily compensate for the height difference within and between cylinder heads and which can be easily controlled in its lateral movement to provide rapid and accurate repositioning without undue strain on the pilot or spindle.